The efficiency of indium phosphide (InP) solar cells is limited by its high surface recombination velocity (approximately 10(exp 7) cm/s). This might be reduced by a wide-bandgap window layer. The performance of InP solar cells with wide-bandgap (1.8 eV) lattice-mismatched In(0.40)Al(0.60)As as a window layer was calculated. Because the required window layer thickness is less than the critical layer thickness, growth of strained (pseudomorphic) layers without interfacial misfit dislocations should be possible. Calculations using the PC-lD numerical code showed that the efficiencies of baseline and optimized p(+)n (p-on-n) cells are increased to more than 22 and 24 percent, (air mass zero (AMO), 25 C), respectively for a lattice-mismatched In(0.40)Al(0.60)As window layer of 10-nm thickness. Currently, most cell development work has been focused on n(+)p (n-on-p) structures although comparatively little improvement has been found for n(+)p cells.